Suzuki Club UK

Yes, I see. That is odd! And I can see why it'd work in a clock, but I really doubt anyone's going to use something like that in a diff.

Besides, I should imagine that the spacing on the other gear would have to be the same (otherwise the teeth would clash and you'd end up with the gears fouling each other and everything grinding to a halt), and therefore the number of teeth would be lower on that gear too - proportionally lower - meaning that the ratio would stay the same.

I'm still not going mental then!

Crikey, this is getting heavy now.

The original question was:-

Q1. Help me settle an argument - is a diff ratio a function of the size of the crownwheel versus the size of the pinion?

A1. It is the number of teeth that determines the ratio, not diameters. Standard gears have a calculated diameter, but gear tooth geometry can be "corrected", for many reasons, but mainly small number of teeth, pinions are corrected positively for strength purposes, so this increases the outside diameter of the blank. Generally the wheel is corrected negatively, but not always. This would make the diameter smaller, so the proportion of the diameter of the gears would alter but the ratio stays the same as the number of teeth is the same.


Q2. And can you make the teeth on a pinion smaller without making the pinion smaller *or* increasing the number of teeth on the pinion?

A2. Yes, but you need to modify the wheel geometry to suit, you would alter the pitch. Then as you later said "I mean the teeth on the pinion. I've been told that a pinion is always the same size, and that the teeth are simply made smaller without changing the number of teeth when mated to a crownwheel with more (and therefore smaller) teeth.

I argued that it's impossible to make the teeth smaller without either making the pinion smaller, or increasing the number of teeth (to take up the excess surface area created by making the teeth smaller). If you had a 10/50 ratio and changed the gears for a 10/52 set for the same diff, the pitch would change, the pinion would generally be slightly smaller, and the wheel larger, but you wouldn't be able to fit the 10t pinion, even in an emergency to the other set, as the geometry would be VERY wrong.

Your analogy to the fair ground ride is good, but those diameters (tyre and drum), are similar to the pitch diameters of gears, which is a point somewhere down the tooth. Usually about mid tooth depth, not the outside diameters.

All this said, I hope you won the argument

I've got a headache now..

Ladaman wrote:All this said, I hope you won the argument

Cheers, that all makes sense. And yes, I was ignoring the fact that the "circumference" of a gear (for the purposes of working out ratios) is not measured at the points of the teeth, but rather at the mid-point where the gears intersect when the gears are meshed (as you say, about halfway down the tooth) because it didn't seem necessary to over-complicate the analogy at the time.

I had decided to let sleeping dogs lie, but then he comes up to me and says he can see my point now - argument over (although I don't know who "won"...).

I love a good "spirited discussion"...

Pretty no on subject idea but. Ashcroft transmissions have a pretty cool ratio calculator online.